1. Field of the Invention
The present invention relates generally to an apparatus and methods for determining the start of combustion in an internal combustion engine.
2. Description of Related Art
Relatively recently, because of the increased regulatory pressure for fuel efficient and low emissions vehicles, some engine designers have directed their efforts to one type of an internal combustion engine which utilizes premixed charge compression ignition (PCCI). Researchers have used various other names in referencing PCCI combustion including homogeneous charge compression ignition (HCCI) as well as others such as xe2x80x9cATACxe2x80x9d which stands for xe2x80x9cActive Thermo-Atmosphere Combustion.xe2x80x9d (SAE Technical Paper No. 790501, Feb. 26-Mar. 2, 1979), xe2x80x9cTSxe2x80x9d which stands for xe2x80x9cToyota-Sokenxe2x80x9d (SAE Technical Paper No. 790840, Sep. 10-13, 1979), and xe2x80x9cCIHCxe2x80x9d which stands for xe2x80x9ccompression-ignited homogeneous chargexe2x80x9d (SAE Paper No. 830264, 1983). All of these terms are hereinafter collectively referred to as PCCI.
Generally, conventional internal combustion engines are either a diesel or a spark ignited engine, each, to a large extent, controlling the start of combustion (SOC) which has been found to be critical in both efficiency and emissions of internal combustion engines. Initially, it should be understood that SOC refers to combustion phasing where the energy of the fuel is released. In this regard, SOC is commonly referred to as the point in time or crank angle at which a charge within the cylinder begins to ignite or rapidly combusts. The diesel engine controls the SOC by the timing of fuel injection while a spark ignited engine controls the SOC by the spark timing. The major advantage that a spark-ignited natural gas or gasoline engine has over a diesel engine is its ability to achieve extremely low NOx and particulate emissions levels. The major advantage that diesel engines have over premixed charge spark ignited engines is in its higher thermal efficiency. One key reason for the higher efficiency of diesel engines is its ability to use higher compression ratios than premixed charge spark ignited engines since the compression ratio in premixed charge spark ignited engine must be kept relatively low to avoid knock. A second key reason for the higher efficiency of diesel engines lies in the ability to control the diesel engine""s power output without a throttle. This eliminates the throttling losses of premixed charge spark ignited engines and results in significantly higher efficiency at part load for diesel engines. Typical diesel engines, however, cannot achieve the very low NOx and particulate emissions levels which are possible with premixed charge spark ignited engines. Due to the mixing controlled nature of diesel combustion, a large fraction of the fuel exists at a very fuel rich equivalence ratio which is known to lead to particulate emissions. Premixed charge spark ignited engines, on the other hand, have nearly homogeneous air fuel mixtures which tend to be either lean or close to stoichiometric, resulting in very low particulate emissions. A second consideration is that the mixing controlled combustion in diesel engines occurs when the fuel and air exist at a near stoichiometric equivalence ratio which leads to high temperatures. The high temperatures, in turn, cause high NOx emissions. Lean burn premixed charge spark ignited engines, on the other hand, burn their fuel at much leaner equivalence ratios which results in significantly lower temperatures leading to much lower NOx emissions. Stoichiometric premixed charge spark ignited engines, on the other hand, have high NOx emissions due to the high flame temperatures resulting from stoichiometric combustion. However, the virtually oxygen free exhaust allows the NOx emissions to be reduced to very low levels with a three-way catalyst.
Unlike these conventional internal combustion engines, engines operating on PCCI principles rely on autoignition of a relatively well premixed fuel/air mixture to initiate combustion. More specifically, in PCCI engines, the fuel and air are mixed in the intake port or in the cylinder, long before ignition occurs. The extent of the mixture may be varied depending on the combustion characteristics desired. Some engines may be designed and/or operated to ensure the fuel and air are mixed into a homogeneous, or nearly homogeneous, state. Also, an engine may be specifically designed and/or operated to create a somewhat less homogeneous charge having a small degree of stratification. In both instances, the mixture exists in a premixed state well before ignition occurs and is compressed until the mixture autoignites. Thus, PCCI combustion event is characterized in that: 1) the majority of the fuel is sufficiently premixed with the air to form a combustible mixture throughout the charge at the time of ignition; and 2) ignition is initiated by compression ignition. In addition, PCCI combustion is also preferably characterized in that most of the mixture is significantly leaner than stoichiometric to advantageously reduce emissions, unlike the typical diesel engine cycle in which a large portion, or all, of the mixture exists in a rich state during combustion. Because an engine operating on PCCI combustion principles has the potential for providing the excellent fuel economy of the diesel engine while providing NOx and particulate emissions levels that are much lower than that of current spark-ignited engine, it has recently been the subject of extensive research and development.
It has been recognized by the applicants of the present invention that the key to producing a commercially viable PCCI engine lies in the control of the combustion history of subsequent or future combustion events in such a manner so as to result in extremely low nitrous oxide (NOx) emissions combined with very good overall efficiency, combustion noise control and with acceptable cylinder pressure. The combustion history may include the time at which combustion occurs (start of combustion), the rate of combustion (heat release rate), the duration of combustion and/or the completeness of combustion. Applicants have determined that the combustion history, and especially the start of combustion (SOC), is sensitive to, and varies depending on, a variety of factors including changes in load and ambient conditions.
In addition, it has also been found by the present applicants that for efficient, low emission PCCI combustion, it is important to have the phasing of combustion or SOC occur properly at an appropriate crank angle during the engine cycle. If combustion starts too early, cylinder pressures will be excessively high and efficiency will suffer. If combustion starts too late, then combustion will be incomplete resulting in poor HC emissions, poor efficiency, high carbon monoxide (CO) emissions, and poor stability. It has further been found that the timing of the start of combustion (SOC) and the combustion rate (therefore combustion duration) in a PCCI engine primarily depend on various combustion history values such as the temperature history; the pressure history; fuel autoignition properties (e.g. octane/methane rating or activation energy); and trapped cylinder charge air composition (oxygen content, EGR, humidity, equivalence ratio etc.).
A premixed charge compression ignition engine with optimal combustion control with various control features for controlling SOC and the combustion rate is disclosed in the patent application Ser. No. 08/916,437 filed on Aug. 22, 1997, currently assigned to the Assignee of the present invention. This application has also been published as International Patent Application No. PCTUS97/14815. As disclosed in the ""437 application, it has been found that active control is desirable to maintain the SOC and duration of combustion at the desired location of the crank shaft and at the desired duration, respectively, to achieve effective, efficient PCCI combustion with high efficiency and low NOx emissions.
More specifically, the ""437 application discloses a PCCI engine comprising an engine body, a combustion chamber formed in the engine body and combustion history control system for controlling a combustion history of future combustion events to reduce emissions and optimize efficiency. The combustion history control system includes at least one of a temperature control system for varying the temperature of the mixture of fuel and air, a pressure control system for varying the pressure of the mixture, an equivalence ratio control system for varying an equivalence ratio of the mixture and a mixture autoignition property control system for varying an autoignition property of the mixture. The engine further includes an operating condition detecting device for detecting an engine operating condition indicative of the combustion history and generating an engine operating condition signal indicative of the engine operating condition, and a processor for receiving the engine operating condition signal, determining a combustion history value based on the engine operating condition signal, and generating one or more control signals based on the combustion history value. The one or more control signals are used to control at least one of the temperature control system, the pressure control system, the equivalence ratio control system and the mixture autoignition property control system to variably control the combustion history of future combustion events. As also disclosed in the ""437 application, the engine operating condition detecting device may include a start of combustion (SOC) sensor for sensing the start of combustion and generating a start of combustion signal. Also, the combustion history value may be determined based on the start of combustion signal. And as further disclosed, the engine operating condition detecting device may also be a cylinder pressure sensor.
However, although SOC can be determined using conventional methods, more effective and efficient apparatus and methods for accurately determining start of combustion are needed to allow more precise control of combustion. Moreover, such an apparatus and methods are required for use in various engines including spark-ignited diesel and alternative fuel engines as well as PCCI engines to allow more effective and accurate control of combustion.
Therefore, there exists an unfulfilled need for an effective and efficient apparatus and method for determining the start of combustion in an internal combustion engine to allow establishing a closed loop control of combustion. Furthermore, there also exists an unfulfilled need for a method of ensuring the integrity of the SOC determinations and a method of determining any occurrence of misfire and existence of retarded timing in the internal combustion engine.
In view of the foregoing, it is an object of the present invention to provide an effective apparatus and method for determining the start of combustion in an internal combustion engine.
A second object of the present invention is to permit active control of combustion by providing an efficient method for determining the occurrence of start of combustion.
A third object of the present invention is to provide a verification test for ensuring the integrity of the SOC determinations.
Yet another object of the present invention is to provide a method for determining any occurrence of misfire and existence of retarded timing in the internal combustion engine.
In order to permit effective control of the start of combustion (SOC), the timing of the start of combustion should be determined and monitored. It has been found by the present applicants that in certain applications of internal combustion engines such as PCCI engines, SOC may be determined based on a predetermined, well-defined knock/ignition line defining various temperatures and pressures at which combustion will occur. However, applicants"" experimental tests show that the SOC is not perfectly repeatable. The applicants have found that SOC varies between each combustion cycle of each cylinder even at same operating conditions over a long period of time, and also varies between each of the cylinders of the engine. This variation in the SOC, between sequential combustion events in a single cylinder engine and between cylinders in a multi-cylinder engine, is due to the sensitivity of PCCI combustion to the pressure and temperature history leading up to the particular combustion event. However, as disclosed in the ""437 application, the PCCI engine may be operated to achieve optimum PCCI combustion despite this sensitivity by providing it with features to control various categories of control.
The present inventors have found that one way to effectively implement the previous disclosed optimal combustion control is by establishing a closed loop control using the start of combustion (SOC) as an input to the combustion history control system which may then control any of the control systems to affect the desired SOC. This closed loop control would allow ideal combustion in an internal combustion engine such as ideal PCCI combustion during the operation of the PCCI engine.
In order to establish such a closed loop control, the SOC must be accurately determined and monitored in any given cylinder. Although SOC can be determined using conventional methods, more effective and efficient apparatus and methods for accurately determining start of combustion are needed to allow more precise control of combustion. In addition, there are also no known effective methods for checking the integrity of the SOC determinations. Furthermore, there are also no known effective methods for checking for any misfire in the PCCI engine.
In accordance with preferred embodiments of the present invention, these and other objects are obtained by methods for determining the start of combustion (SOC) in an internal combustion engine including the steps of obtaining cylinder pressure data (P), processing the cylinder pressure data (P) into a processed pressure value indicative of the occurrence of SOC, comparing the processed pressure value to a predetermined threshold value, if the processed pressure value crosses the predetermined threshold value, determining that SOC has occurred, and calculating a crank shaft location at which the predetermined threshold value was crossed by the processed pressure value thereby identifying the crank shaft location at which SOC occurred.
In accordance with alternative embodiments of the present invention, the method may also be provided with optional steps including a verification step, a windowing step, a filtering step, determining occurrence of misfire, and determining occurrence of retarded timing. In this regard, the verification step may be included to ensure integrity of the cylinder pressure data (P). In one embodiment, the verification step includes monitoring continuity of the cylinder pressure data (P) across a bottom dead center boundary of a reciprocally mounted piston in the cylinder. Preferably, a fault signal is provided if the cylinder pressure data across the bottom dead center boundary is not continuous. Alternatively, the verification step may include comparing P measured when a reciprocally mounted piston in the cylinder is at a top dead center position with P measured when the piston is at a bottom dead center position. In this embodiment, the fault signal is provided if the P measured at the top dead center is not substantially greater than the P measured at bottom dead center position.
In one embodiment where the optional step of windowing is provided, the cylinder pressure data (P) is synchronized to an angle of a crank shaft, and the cylinder pressure data (P) is windowed within a predetermined crank shaft angle window which is inclusive of the start of combustion (SOC). In this regard, the predetermined crank shaft angle window may be between xe2x88x92180 and +180 crank shaft degrees from the SOC but is preferably between xe2x88x9210 and +30 crank shaft degrees.
In another embodiment where the optional step of filtering is provided, at least one of the cylinder pressure data (P) and the processed pressure value and may be attained by an analog filter and/or a digital filter. In this regard, the filter may have a cutoff frequency inversely proportional to the bore size of the cylinder of the engine or the bowl size of a piston received within the cylinder of the engine. The filter may alternatively, have a cutoff frequency of approximately 11,000 Hz/(diameter of the cylinder in inches).
In still another embodiment where the occurrence of misfire is optionally determined, the misfire may be determined to have occurred when the processed pressure value does not cross the predetermined threshold within a predetermined crank shaft angle window. A misfire fault signal may be provided upon determining occurrence of misfire, and the SOC crank shaft location may be defined to be a predetermined angle (such as 50 degrees after top dead center position of a piston reciprocally mounted in the cylinder) upon providing said misfire fault signal. In alternative embodiments, the cylinder pressure data (P) may be synchronized to angle of a crank shaft, and the occurrence of misfire may be determined by calculation of a pressure ratio (PR) defined as PR(xcex8)=P(xcex8)/P(xe2x88x92xcex8), where xcex8 is a crank angle between 10xc2x0 to 45xc2x0, wherein if PR(xcex8) is not greater than a reference level which may be one, a misfire is determined to have occurred.
In still another embodiment where the existence of retarded timing is optionally determined, the cylinder pressure data (P) may be synchronized to angle of a crank shaft, and the existence of retarded timing is determined by comparing a pressure ratio (PR) to a predetermined expected PR value, the pressure ratio being defined as PR(xcex8)=P(xcex8)/P(xe2x88x92xcex8), where xcex8 is a crank angle between 10xc2x0 to 45xc2x0.
In addition, in accordance with various embodiments of the present method, the processed pressure value may be the cylinder pressure itself, a rate of change in the cylinder pressure, an acceleration of the change in the cylinder pressure, or a cumulative heat release value. More specifically, in one embodiment of the present invention, the processed pressure value may be an isentropic compression pressure value. In this regard, the cylinder pressure data (P) may be synchronized to angle of a crank shaft, and the existence of retarded timing may be determined by comparing the cylinder pressure to a predetermined expected isentropic compression pressure value, where if the cylinder pressure is less than the predetermined expected isentropic compression pressure value, determining existence of retarded timing. Moreover, the existence of retarded timing may be determined by comparing a ratio of peak cylinder pressure (PCP) and a pressure earlier in a combustion cycle P(xe2x88x92xcex8) to a predetermined expected value or by comparing a pressure ratio (PR) to a predetermined expected PR value, the pressure ratio being defined as PR(xcex8)=P(xcex8)/P(xe2x88x92xcex8), where xcex8 is a crank angle between 10xc2x0 to 45xc2x0.
In another embodiment of the present method, the cylinder pressure data (P) may be synchronized to angle of a crank shaft, and the cylinder pressure data (P) is processed into a processed pressure value which includes calculation of a pressure change rate. In this regard, the pressure change rate may be dP/dxcex8 value indicative of rate of change in pressure in the cylinder with respect to an angle of the crank shaft, or may be dP/dt value indicative of rate of change in pressure in the cylinder with respect to time. In addition, the step of processing the cylinder pressure data (P) may further include determining a peak pressure change rate (PPCR) which may then be compared to the predetermined threshold value to determine the SOC. Furthermore, the peak pressure change rate (PPCR) may be compared with a minimum pressure change rate to determine occurrence of misfire if the PPCR is substantially similar to the minimum pressure change rate, or to determine the existence of advanced combustion if the PPCR is substantially greater than the minimum pressure change rate.
In yet another embodiment of the present method, the cylinder pressure data (P) may be synchronized to angle of a crank shaft, and the cylinder pressure data (P) is processed into a processed pressure value which includes calculation of a pressure change acceleration rate. In this regard, the pressure change acceleration rate may be d2P/dxcex82 indicative of acceleration of pressure in the cylinder with respect to angle of the crank shaft, or d2P/dt2 value indicative of acceleration of pressure in the cylinder with respect to time. The pressure change acceleration rate may be obtained by processing the cylinder pressure data (P) utilizing a software program executed by a processor, or by processing the cylinder pressure data (P) utilizing a differentiator circuit having an operational amplifier, such as by using two differentiator circuits in series. Additional processing of the pressure change acceleration rate may be provided such as by using a filter, a level shifter, or a comparator, and the step of comparing the processed pressure value to the predetermined threshold value may be attained utilizing a comparator circuit.
In still another embodiment of the present invention, the cylinder pressure data (P) may be synchronized to angle of a crank shaft, and the step of processing the cylinder pressure data (P) into a processed pressure value includes the step of calculating an apparent heat release rate (AHR (i)) value in the cylinder based on the cylinder pressure data P, where the AHRR (i) value is indicative of the rate of heat released in the cylinder with respect to angle of the crank shaft. In this regard, a pressure change rate (dP/dxcex8 (i)) value indicative of rate of change in pressure in the cylinder with respect to angle of the crank shaft may also be calculated where the apparent heat release rate (AHRR(i)) value is calculated based on the dP/dxcex8 (i) value. Preferably, a cumulative heat release (CHR(i)) value in the cylinder is calculated based on the AHRR(i) value, the CHR(i) value indicative of cumulative heat released in the cylinder with respect to angle of the crank shaft. The CHR(i) value may then be compared to the predetermined threshold value and if the calculated CHR(i) value crosses the predetermined threshold value, the occurrence of the SOC may be determined. Moreover, the angle of the crank shaft at which SOC occurred can be determined by calculating the angle of the crank shaft at which the predetermined threshold value is crossed.
In accordance with another aspect of the present invention, the above noted objects and others are obtained by an apparatus for determining a start of combustion (SOC) in a cylinder of an engine including a pressure sensing means and a processor. In particular, the pressure sensing means senses pressure in the cylinder and provides a pressure signal indicative of the pressure in the cylinder. The processor receives the pressure signal from the pressure sensing means, processes the pressure signal into a processed pressure signal, compares the processed pressure signal to a predetermined threshold value, determines that SOC has occurred if the processed pressure signal crosses the predetermined threshold value, and calculates crank shaft location at which SOC occurred. In one embodiment of the present invention, the processor may include an electronic control unit (ECU). The pressure sensing means may be one or more of a pressure sensor, accelerometer, ion probe, optical diagnostic, strain gage, load washer, fast thermocouple, torque sensor, RPM sensor and emissions sensor. However, preferably, the pressure sensing means is a pressure sensor. The apparatus in accordance with the present invention may be provided in a diesel engine, a spark ignited engine, a flexible fuel engine, or a premixed charge compression ignition (PCCI) engine, but is especially advantageous when applied to a premixed charge compression ignition (PCCI) engine. In this regard, if the engine to which the present invention is applied includes a combustion history control system, the processor may be used to variably control the combustion history control system based on the calculated SOC.
These and other objects, features and advantages of the present invention will become more apparent from the following detailed description of the preferred embodiments of the invention when viewed in conjunction with the accompanying drawings.